This invention relates to a particle beam apparatus for irradiating a particle beam on a specimen and detecting backscattered and secondary particles released at the specimen, and more particularly, to a particle beam apparatus of low voltage application to achieve an improved detection efficiency of backscattered and secondary particles.
In a particle beam apparatus, low voltage application is extremely important in the imaging of sensitive and non-conductive specimens. Because of the low energy (typically lower than 5 keV), resulting in low energy dissipation, sensitive specimens are not damaged. Insulating specimens can be imaged without distortion or disturbance, because insulators have a secondary electron yield of about 1 in the low energy range, which avoids or minimizes charging effects during the exposure with the particle beam. Low voltage microscopy, consequently, has a great importance for the dimensional measurement and the inspection of device structures in semiconductor manufacturing processes.
Presently high resolution low voltage microscopes are used for the above mentioned applications. High performance microscopes use a combined electrostatic-magnetic immersion lens as final objective lens. By using the immersion principle, the primary beam path is at high energies. The final low beam energy is generated by deceleration in the objective lens, just in front of the specimen. By applying such intermediate beam acceleration concepts, the electron-electron interaction inside the column, which broadens the beam and consequently decreases the resolution, can be significantly reduced.
The secondary particles released at the specimen by the primary particle beam can be detected by a detector located in front of the objective lens. The arrangement of an in-lens or pre-lens detector has the advantage that the specimen can be located very close to the lens, resulting in a short working distance and correspondingly in a short focal length of the objective lens. A short focal length yields low chromatic and spherical aberration coefficients of the objective lens, which means high optical performance for the low voltage application.
The high performance low voltage arrangements according to the state of the art show a good optical performance, which even can be improved by the application of objective lenses using a combined electrostatic retarding field and magnetic lens. Those arrangements, however, have a drawback in secondary particle detection efficiency. Since the released particles are accelerated by the retarding field of the primary particles, their energy is high and similar to the primary particle energy. Consequently, their behavior is also similar to that of the primary particle beam. Accordingly, the released particle detection is difficult and not very efficient. Therefore, state of the art solutions either use coaxial detectors with small holes for the penetration of the primary beam (EP-B-0 333 018) or means for separation of the primary and the released particle beam (U.S. Pat. No. 5,422,486).
EP-A-1 022 766 proposes a particle beam apparatus, in which, by applying first and second means to decelerate and accelerate the primary particle beam in the region of the detection system, the backscattered and/or secondary particles are decelerated to their original energy distribution by applying a potential in the region of the detector which is close to the potential of the specimen.
Although this known arrangement improves the backscattered and secondary particle detection efficiency, there is still the problem that backscattered particles and secondary particles released at the specimen have different energies. Therefore, it is difficult to detect both kinds of particles in a high efficient manner.
U.S. Pat. No. 5,644,132 discloses a particle beam apparatus for charge-free high resolution imaging and measurement of topographic and material features on a specimen. A particle beam source provides a primary particle beam along a primary beam axis, said primary beam impinging on the specimen so as to release backscattered and secondary electrons. An objective lens is focussing said electrons so as to provide a radial dispersion of said electrons relative to said primary beam axis, said radial dispersion of electrons including an inner annulus of backscattered electrons and an outer annulus of secondary electrons. Furthermore, the apparatus comprises a backscattered electron detector for detecting said inner annulus of backscattered electrons and a secondary electron detector for detecting said outer annulus of secondary electrons. The backscattered electron detector is an electron multiplier.
Furthermore, U.S. Pat. No. 4,308,457 discloses a device for the detection of backscattered electrons emitted by a specimen in an electron microscope, comprising a converter for converting backscattered electrons emitted by the specimen into converted secondary electrons.
It is an object of the invention to provide a particle beam apparatus with an improved detection efficiency of backscattered and secondary particles released at the specimen.
The particle beam apparatus according to the invention comprises:
a source for generating a primary particle beam,
means for focussing the primary particle beam onto a specimen,
a detection system for detecting particles released at the specimen, e.g. backscattered and/or secondary and/or Auger electrons, said detection system being located between the source and said focussing means, said detection system comprises at least one detector,
first means to accelerate the primary particle beam to a first energy,
first means to decelerate the primary particle beam before the detection system from the first energy to a second lower energy,
second means to accelerate the primary particle beam after the detection system from the second energy to a third higher energy, and
second means to decelerate the primary particle beam from the third energy to a final beam energy.
The detection system further comprises a converter to convert particles released at the specimen into converted secondary particles which will be detected by the detector.
By providing a deceleration area for the particles released at the specimen in the region of the detection system in combination with a converter for backscattered particles, it is possible to improve the detection efficiency for both, backscattered and secondary, particles.
According to a preferred embodiment, there is a first control electrode and/or a second control electrode, wherein the first and/or second control electrode can be applied with a suitable voltage in order to suppress a part of the particles released at the specimen. With the help of these control electrodes, it is possible to obtain a secondary particle image or a backscattered particle image of the specimen. It is also possible to generate an image, which is based on both kinds of particles.